What principle states that the just noticeable difference of a stimulus is a constant proportion despite variations?

The principle that the just noticeable difference (JND) of a stimulus is a constant proportion, regardless of the absolute level of the stimulus, is known as Weber's Law. This law was formulated by Ernst Heinrich Weber in the 19th century and fundamentally shaped our understanding of sensory perception. According to Weber's Law, if you have a certain weight, for instance, there will be a constant ratio that defines the minimum weight difference needed for a person to notice a change. This means that, for example, if you have two weights, a person will only be able to tell the difference if the additional weight meets a certain percentage of the initial weight, illustrating that the perception of difference is relative rather than absolute.

In contrast, Fechner's Law builds upon Weber's Law and relates sensory perception to physical stimulus intensity, suggesting that perceived intensity increases logarithmically with stimulus intensity. Stevens' Power Law suggests a power relationship between the intensity of a stimulus and its perceived intensity, and Threshold Theory pertains to the minimum level of stimulus intensity needed for detection. While these principles address different aspects of perception and measurement, it is Weber's Law that specifically articulates the constant proportionality involved in just noticeable differences.